Here is something not in the release notes that people may want to watch out for: The format of the session has changed, the FlashHash class to be precise, so trying to load a browser session created on Rails 3.0 will give a marshal load error on 3.1 if it had anything in the flash.

This means you will want to change your SESSION_KEY variable to expire all sessions, otherwise users that are logged in while you upgrade can get stuck until they clear their cookies. (So I postponed the upgrade to the weekend...)